Research and development activities for realizing over-all high-functional optical communications are being aggressively promoted in the optical communications network field. Prior WDM systems exhibit narrowband wavelength intervals from about 200 GHz-50 GHz and from about 100 GHz to about 25 GHz, within which several wave lengths may be selected. The number of selected wavelengths available for optical communications may increase from the order of tens of wavelength values to the order of hundreds of wavelength values. In order to effectuate these changes, there is a need for a wavelength variable light source system capable of dynamically changing wavelength and light output intensity of the output signal light.
FIG. 17 is a block diagram illustrating a configuration of a prior art wavelength variable light source system. The wavelength variable light source system 211 includes a wavelength variable light source 212 and a control circuit 214 for controlling the wavelength variable light source 212 in accordance with instructions from a higher-level device 213.
As a signal light device of the wavelength variable light source 212, a semiconductor laser 215 may be used. The semiconducting laser may be a DFB (Distributed Feedback) laser that controls wavelength of the output signal light by controlling device temperature or it may be a DBR (Distributed Bragg Reflector) laser that controls wavelength of the output signal light by controlling driving current.
The wavelength variable light source 212 comprises the semiconductor laser 215 and a thermoelectric cooler (TEC) 216 for controlling the temperature of the semiconductor laser 215. The semiconductor laser 215 is mounted on the TEC 216. A TEC temperature detector 217 is installed on the TEC 216. A beam splitter (BS) 218 for splitting part of the output signal light from the semiconductor laser 215 is provided on an optical axis of the semiconductor laser 215 and a wavelength filter 219 that transmits only light of specific wavelength and a photodiode (PD) 220 that detects the transmitted light are provided on a split-side optical axis of the BS 218.
The control circuit 214 has; a current monitoring circuit 221 for monitoring the driving current of the semiconductor laser 215, a temperature monitoring circuit 222 for monitoring the temperature of the semiconductor laser 215 based on the TEC temperature detector 217, an output signal light monitoring circuit 223 for monitoring intensity of the signal light of a specific wavelength component contained in the output signal light of the semiconductor laser 215 based on an output of the PD 220, a CPU (Central Processing Unit) 224 that outputs a LD (Light Diode) driving current control signal for controlling the driving current of the semiconductor laser 215, a TEC temperature control signal for controlling the temperature of the semiconductor laser 215 based on outputs of these monitoring circuits 221, 222, and 223, a LD driving circuit 225 for driving the semiconductor laser 215 corresponding to the LD driving current control signal, and a TEC driving circuit 226 for driving the TEC 216 corresponding to the TEC temperature control signal.
Output signals of the monitoring circuits 221, 222, and 223 are converted into digital signals respectively by ADCs (Analog Digital Converters) 227, 228, and 229; and are transmitted to the CPU 224. The LD driving current control signal and the TEC temperature control signal outputted out of the CPU 224 are converted into analog signals respectively by DACs (Digital Analog Converters) 230 and 231, and are inputted to the LD driving circuit 225 and the TEC driving circuit 226. The control circuit 214 controls the wavelength variable light source 212 so that the wavelength of the output signal light becomes constant by performing a feedback control while monitoring the driving current, temperature, and intensity of the signal light of the semiconductor laser 215 so that differences between those values and their target values are eliminated. It is noted that the content of the control is different when the semiconductor laser 215 is the DFB laser and when it is the DBR laser.
In the case of the wavelength variable light source 212 when the DFB laser is used as the semiconductor laser 215, the temperature and signal light output intensity of the semiconductor laser 215 are monitored. Information from this monitoring are used to create a feedback control for determining a command value of the TEC temperature. The command temperature value is used such that differences between actual values and target values are eliminated to control the wavelength of the output signal light. The CPU 224 also simultaneously carries out a feedback control for maintaining a constant driving current of the semiconductor laser 215 based on the output value of the current monitoring circuit 221. The current feedback is independent of the feedback control for adjusting the temperature.
In the case of the wavelength variable light source 212 when the DBR laser is used as the semiconductor laser 215, the driving current and signal light output intensity of the semiconductor laser 215 are monitored. A feedback control for determining a command value of the driving current is established so that the difference between the actual value and target value of the current is eliminated in order to control the output signal light. The CPU 224 also simultaneously carries out the feedback control for maintaining the temperature of the semiconductor laser 215 constant based on the output value of the TEC temperature detector 217 independently of the feedback control for adjusting the current.
Applicants incorporate by reference Japanese Patent Application Laid-open No. Hei. 9-219554 which describes a wavelength control technology for controlling wavelength of output signal light by detecting intensity of light signal by inputting light from the semiconductor laser to a wavelength filter whose transmission characteristics is inverse and by adjusting temperature of the semiconductor laser corresponding to a difference of the detected intensity of the light signal. Applicants also incorporate by reference Japanese Patent Application Laid-open No. Hei. 11-220198 which describes a wavelength control technology of controlling wavelength of output signal light by controlling a movement of a reflecting surface in an outside resonance section based on absorption wavelength detected by a wavelength detecting section.
In order to build a high functional optical communication network, it is desirable to increase a wavelength variable range and a number of wavelengths. Further, it is desirable to increase the variability of the intensity of output signal by a wavelength variable light source system. Typically, it had been required to mount a plurality of semiconductor lasers and a SOA (Semiconductor Optical Amplifier) within the wavelength variable light source system.
However, with the advance of the functions of the wavelength variable light source systems, problems related to influences of heat and light scatter within the wavelength variable light source system and their effects on output characteristics have outpaced the needs for setting accuracy and stability of the output wavelength and intensity of the optical output.